How downbeat and unfunny can a movie be before it stops being a comedy altogether? Don’t ask Melissa McCarthy and Ben Falcone, the very talented pair behind the very bad movie Tammy. With their first joint outing as co-writers and co-producers, the husband-and-wife team flop-sweat their way over that line in the opening scene. They occasionally and briefly manage to find their way back to the right side of it by dint of great casting and McCarthy’s weirdly abrasive charm, but Tammy is mostly a colossal miscalculation of the most basic comic equations.

To be clear, I’m not talking about dark comedy here; I laughed myself silly during Killer Joe, American Psycho, and Super, so understand that Tammy exists in a very different realm of discomfort and unpleasantness. It’s a lumbering, aimless belly flop of a movie that fails miserably in spite of a great cast and a solid, if not exactly original, premise.

Comedy is every bit as personal as horror, of course, and some will say that the key to enjoying Tammy is one’s degree of fondness for, or at least tolerance of, McCarthy’s lowbrow shtick. Allow me to make an embarrassing confession, then: I watch Mike & Molly, and I laugh a lot while I do it. To offer a couple more McCarthy-specific bellwethers, The Heat was one of my favorite movies of last year, while I found Identity Thief to be a pretty miserable experience. So I like the actress very much, but even I have my limits, and Tammy sprints past them right out of the gate.

The basic elements of a solid comedy are in place. McCarthy heads up a cast that includes Susan Sarandon, Kathy Bates, Toni Collette, and Dan Aykroyd, and there’s a lot of potential in the basic set-up, which follows a recently fired fast-food worker and her alcoholic granny as they Thelma-and-Louise their way across a grimy middle-American landscape of greasy spoons and roach motels. But it’s clear from the title that Tammy is no ensemble piece, and that its success—or, more accurately, its failure—rests squarely on how it uses, or misuses, its star. And since McCarthy is a screenwriter and producer here as well as the main attraction, well, there’s not really anyone else to blame.

McCarthy stars as the eponymous character, and in the first few minutes Tammy finds herself fired from a dead-end job and jilted by a cheating husband. Her life is as broken-down as her car and the remedy is clear: road trip! Except the journey Tammy undertakes is very clearly concerned more with the “from” than the “to”; she and Pearl, her sexy and hard-drinking grandmother (Sarandon), set out for Niagara Falls, but we know they’re just trying to put as much road as possible between Pearl’s Cadillac and their sad, lonely lives. Over the course of their journey, we learn that Pearl is diabetic and very ill and that Tammy is her own worst enemy: petulant, self-involved, childish, and kind of dumb.

Funny, right? Most of the comic set pieces revolve around Tammy’s repeated humiliation and her and Pearl’s uncomfortable revelations about past indiscretions. Tammy makes a habit of wild, unmeasured tonal shifts as it meanders between poorly staged slapstick and downbeat drama. The one constant, as you’ll guess after the film’s opening scenes, is that Tammy rarely treats its pathetic main character as anything more than a punchline. There’s a brief respite from Tammy and Pearl’s depressing downward spiral when they hook up with Lenore (Bates), the wealthy and pyromaniacal leader of an upper-class lesbian tribe, but even that leg of their journey soon descends into a sort of poorly conceived trailer-park despair. (Though, in all fairness, it does set up the movie’s biggest laugh—Sarandon really is very funny.)

Comedies famously benefit from tight, or at least pointed, direction, and Falcone just doesn’t have that sense of timing and control yet. It’s stunning how little these two comedy veterans—Falcone has appeared in dozens of movies and TV shows—seem to understand about the basic anatomy of a good gag. Tammy also grossly underestimates its star’s abilities, and too often falls back on jokes about her appearance. While The Heat constantly upended expectations about McCarthy’s foul-mouthed, slovenly character, Tammy usually plays those stereotypes at face value.

And that really is a shame, because it indicates that McCarthy herself has yet to understand how very awesome Melissa McCarthy can be when she’s pushed beyond the obvious typecasting that has made her such a bankable star.